Butadiene and isoprene copolymerization were carried out at 50 ℃ in hexane for 8 h using Nd(P507)3/AlH(i.Bu)2/AlEt2Cl catalyst.The catalyst was aging at 60 ℃ for 2 h before initiation.FTIR method was used to determine the microstructure of butadiene isoprene copolymer.The result showed the copolymer was of high cis-1,4 content in both polybutadiene unit(cis-1,4=93.1%-97.7%) and polyisoprene unit(cis-1,4=97.0%-97.5%).DSC determination indicated that the copolymer was of excellent low temperature resistance.The butadiene-isoprene copolymers showed only one glass transition temperature(Tg) from -107.2 ℃ to – 64.4 ℃,which increased with the increasement of the isoprene ratio.The correlation deviated from Fox equation,and could be modified as Tg＝1.03TgIWI+TgBWB.The reactivity rate of the copolymer was calculated by Kelen-Tudos method.It turned out that the reactivity rates of butadiene(r1) and isoprene(r2) were 1.21 and 0.73，respectively.The product of r1 and r2 was close to 1,which indicated the product is a random structure.The 13C NMR spectrum was used to determine the sequence distribution of the copolymer.The peak position in the aliphatic carbon part showed no difference while the composition changed in the copolymer.The compositions of 4 dyads of random butadiene-isoprene copolymers and the number average sequence length of polybutadiene and polyisoprene were calculated based on the peak area of 13C NMR spec-trum.The data were analyzed and examined in use of the statistical formula under Bernoullian model and Markov model.It turns out that all the data are more identical with Markov model than Bernoullian model.Moreover,the copolymer chains with higher Bd content contain longer polybutadiene segments,while the copolymer chains with higher Ip content contain longer polyisoprene segments.